1. Field of the Invention
The present invention relates to a predistorter used in a distortion-compensated amplifier using predistortion technique for compensating for, by a predistortion technique, a distortion caused in an amplifier when an input signal is amplified by the amplifier, and, more specifically, it relates to a predistorter compensating for a nonlinear distortion due to a memory effect caused in an amplifier.
2. Description of the Related Art
For example, in a base station apparatus provided in a mobile communication system adopting Wide-band Code Division Multiple Access (W-CDMA) as a mobile communication technique, radio signals must be transmitted to a physically remote base station apparatus. Therefore, the signals must be largely amplified by an amplifier. However, since an amplifier is an analog device, the input/output characteristic is a nonlinear function. Especially, after the amplification limit called saturation point, the amount of power to be input to the amplifier increases, and substantially constant power is output. The nonlinear output causes nonlinear distortion. Though the signal component in an undesired signal band of a signal sent before amplification can be kept at a low level by a band limiting filter, nonlinear distortion occurs in the signal having passed through the amplifier. Thus, the signal component leaks to the undesired signal band (adjacent channel). For example, since transmission power is high in a base station apparatus as described above, the magnitude of the power leaking to such an adjacent channel is strictly defined. Therefore, a big issue here is how the power leak to an adjacent channel is reduced.
Accordingly, the predistortion technique has been proposed as one of distortion compensation techniques and is going mainstream instead of the feed forward technique because an amplification coefficient has been emphasized recently. The predistortion technique can compensate for distortion of signals output from an amplifier by giving in advance inverse characteristics of the AM-AM conversion and AM-PM conversion, which are nonlinearity of the amplifier, to signals input to the amplifier.
FIG. 13 shows a construction example of an amplifier compensating for a distortion by a predistortion technique (distortion-compensated amplifier using predistortion technique).
A distortion-compensated amplifier using predistortion technique in this example includes a power detecting portion 81, a distortion compensation table 82, a predistorter 83, an amplifying section 84 and a control portion 85.
An example of an operation to be performed by a distortion-compensated amplifier using predistortion technique in this example will be described.
Once a signal is input to the power detecting portion 81, predistorter 83 and control portion 85, the power detecting portion 81 detects the power or amplitude value of the input signal. Then, the power or amplitude value of the input signal is associated as a reference argument of a distortion compensation table 82 acting as a Look Up Table (LUT), using a memory, for example. The distortion compensation table 82 stores a table value for performing distortion compensation by predistortion technique in accordance with the power or amplitude value of a given signal. The distortion compensating table 82 ideally defines an inverse characteristic of nonlinearity of an amplifier (amplifying section 84 in this example) to be distortion-compensated and generally defines a value relating to AM-AM conversion (amplitude) and value relating to AM-PM conversion (phase) using the power or amplitude of the input signal as an index.
The predistorter 83 controls the amplitude and phase of the input signal in accordance with a result of the reference to the distortion compensation table 82. The signal distorted by the predistortion technique in advance is amplified by the amplifying section 84, and an amplified signal without distortion is output from the amplifying section 84.
In order to adapt to an environment such as a temperature variation, the control portion 85 updates details on the distortion compensation table 82 based on the input signal and amplified input signal.
However, in a distortion causing mechanism in an amplifier, the current state may depend on not only an AM-AM characteristic and AM-PM characteristic relating to instant power but also a past state such as the memory effect. Thus, in the construction including the distortion compensating table 82 referred by instant power as an only index, the nonlinear distortion caused by the memory effect cannot be compensated.
Various studies have been made for such a memory effect (refer to JP-A-11-191716 and “Gusuji Hizumi no Eikyo wo Ukeru Zofukuki ni Taisuru Tekiou Predistorter-Gata Hizumi Hosho (Adaptive Predistorter Type Distortion Compensation For Amplifier Influenced by Even Order Distortion)”, Kawaguchi and Akaiwa, Shingakugiho, MW2002-208 (2003-03), p. 63–66), but further developments for suppressing influences due to a memory effect are still being demanded.